There's Apple machines out there which (probably completely arbitrarily)
restrict each output path to a particular head.  This causes us to not
be able to locate the output data needed to power on/off the DP output
correctly.

We fix this by passing in a head index we know is valid (as opposed to
"head 0").
Signed-off-by: NBen Skeggs <bskeggs@redhat.com>